Roughness sensors of the type disclosed in commonly-assigned U.S. Pat. Nos. 3,789,816 issued Feb. 5, 1974 to Taplin et al. on a "Lean Limit Internal Combustion Engine Roughness Control System," and 3,872,846, issued Mar. 25, 1975 to Taplin et al. on an "Exhaust Gas Recirculation Internal Combustion Engine Roughness Control System," the disclosures of such patents being hereby expressly incorporated herein by reference, provide a roughness signal the magnitude of which varies with changes in engine speed over many engine cycles and therefore lags the actual changes in engine speed. Moreover, such prior art sensors require filtering and differentiating of the speed signal to develop a roughness signal in the form of the first and/or higher order derivatives of speed. The differentiating was necessary to create a speed change signal and the filtering was necessary to separate out the driver-induced, and therefore slower varying, changes in engine speed. Finally, the roughness signals developed by the prior art reference sensors were not normalized for engine speed so that a roughness signal of a given magnitude at a low engine speed and a roughness signal of the same magnitude at a high engine speed causes a roughness utilization device to effect the same response. The prior art roughness sensors require consideration of engine-to-engine differences as well as engine noise and ambient conditions.
It is desirable to provide an engine roughness sensor that detects cycle-to-cycle changes in engine speed as might be produced by cycle-to-cycle changes in combustion pressures. It is also desirable to provide such a cycle-to-cycle roughness sensor that produces a cycle-to-cycle roughness signal for the fact that the same change in engine speed at a speed at a high engine speed and a low engine speeddoes not represent the same magnitude of engine roughness. It is furthermore desirable to provide a cycle-to-cycle engine roughness signal using a speed signal already generated for other closed-loop engine control purposes.
As further described in my commonly-assigned U.S. Pat. No. 3,734,068 issued on May 22, 1973 and entitled "Fuel Injection Control System," and other patents related thereto including U.S. Pat. No. 3,919,981 issued Nov. 18, 1975, the disclosures of each of said patents being expressly incorporated herein by reference, trigger pulses produced by a pair of distributor activated reed switches or by a signal induced from the ignition primary produce flip flop intervals varying inversely with engine speed. I have found that the difference between contiguous, immediately prior or subsequent, trigger pulses indicates changes in speed adequate for closedloop engine control purposes. Therefore the same trigger pulses may be used for roughness sensor purposes as are used in my aforementioned patents to correct the width of a fuel injection pulse for cycle-to-cycle changes in engine speed.